The Fata Morgana is a mirage that tranforms a fairly uniform horizon into one which rises vertically above the horizon. This mirage occurs where the air temperature increases with elevation above the surface. Thus, mirages like the Fata Morgana are often seen where warm air rests above a cold surface such as large bodies of water and polar regions.
Source: Essentials of Meteorology by C. Donald Ahrens

Probably the best thing about endeavoring to learn about poetry and poems for myself is that I can choose what catches my interests as opposed to having it forced upon me as it often is in school.This poem is from Birds of Passage, Flight the Third (1873). Longfellow was recognized in his lifetime and I think a new work by him was looked forward to. Bob Blair from The Poet's corner, an ardent admirer writes:

"Part of the Longfellow caricature is of a poet self-consciously poetic (and) Longfellow's Fata Morgana has a lot of archaicconventions in it, but at heart it is a plain complaint about the impossibility of achieving perfection. "

Not only is Fata Mogana known as the legendary sister of King Authur but it's also a term used by Sicilians as the name of a mirage that manifests during certain seasons off the Calabrian coast.

Sources:

Part of the Longfellow caricature is of a poet self-consciously ...: http://www.geocities.com/~bblair/010227f.htm

Public domain text taken from The Poets’ Corner:
http://www.theotherpages.org/poems/longf03.html#8

A recurring short skit series in the hilarious Norwegian TV-program Åpen Post. Actors Harald Eia and Classe Ekman play two anti-media zealots, with their own airtime. Since the participants of Fata Morgana are the only ones not aware of the irony of bringing hardcore anti-TV messages out on TV, they try as hard as they can to disrupt the program with their messages. Sadly for Fata Morgana, they like to be on TV, even though they are supposed to hate it. Each time they discover things like having groupies, free soda in the studio and other small pleasures.
The also try to get the message out by dancing like robots at the end of each skit while singing "We're dancing like robots". Weird.

Sample skit:Harald Eia: Awww, look at the small puppy (holds a plush puppy), think of puppies and a sommer day. Think of your girlfriend walking through a meadow...Classe Ekman: HIROSHIMA!!! NAGASAKI!!! GEORGE BUSH!!! (Pictures of atom explosions flash on the screen.)Harald Eia: That was an example of the duality of media.